accelerometers. Furthermore the format of the acquired datastream is explained

accelerometers. Furthermore the format of the acquired datastream is explained

−

in detail. This document does not cover the physical basics of acceleration or

+

in detail.

+

+

This document does not cover the physical basics of acceleration or

the mathematical details on this subject.

the mathematical details on this subject.

+

Consult the hardware specs at the STMsite for the [http://www.st.com/stonline/products/literature/ds/12726/lis302dl.htm LIS392DL]. It contains an interesting read how this unit functions.

==Axis orientation==

==Axis orientation==

Line 16:

Line 19:

The Z axis is pointing from the display downwards to to the back of the

The Z axis is pointing from the display downwards to to the back of the

openmoko. This applies to both of the sensors.

openmoko. This applies to both of the sensors.

+

(Also see [http://wiki.openmoko.org/wiki/Talk:Technical:Accelerometer_Fundamentals Definition of gmeter readings as force rather than acceleration])

Line 21:

Line 25:

To explain the axis orientation I have created some images for easier

To explain the axis orientation I have created some images for easier

−

understanding.

+

understanding. Note that X and Y are in the same plane as the screen, while the Z arrow is pointing "into the screen", i.e. behind the Neo. The device for the first sensor is ''/dev/input/event2''.

[[Image:Accelerometer_orientation1.png|Axis orientation of the first accelerometer]]

[[Image:Accelerometer_orientation1.png|Axis orientation of the first accelerometer]]

−

===Orientation of the second sensor===

===Orientation of the second sensor===

In contrast to the first sensor the second one is turned 45 degrees around the

In contrast to the first sensor the second one is turned 45 degrees around the

−

Z axis. See the attached image to get a clue about its orientation.

+

Z axis. See the attached image to get a clue about its orientation. Note that X and Y are in the same plane as the screen, while the Z arrow is pointing "into the screen", i.e. behind the Neo. The device for the second sensor is ''/dev/input/event3''.

[[Image:Accelerometer_orientation2.png|Axis orientation of the second accelerometer]]

[[Image:Accelerometer_orientation2.png|Axis orientation of the second accelerometer]]

Line 38:

Line 41:

different input event based file mappings. These device nodes can be found at

different input event based file mappings. These device nodes can be found at

These device nodes can be opened by the default filesystems calls for reading

These device nodes can be opened by the default filesystems calls for reading

−

files. The data can be read from the stream as soon as the sensor measures it.

+

files. The data can be read from the stream as soon as the sensor measures it. Note that since the data is exported as normal files you can easily e.g. use netcat to stream the accelerometer data over bluetooth to your laptop to a control a game running on your laptop.

−

+

==Data structure==

==Data structure==

Line 53:

Line 63:

system which exports the following data structure:

system which exports the following data structure:

+

<pre>

struct input_event {

struct input_event {

struct timeval time;

struct timeval time;

Line 59:

Line 70:

__s32 value;

__s32 value;

};

};

+

</pre>

The data is written to the stream message by message. Therefore the minimal

The data is written to the stream message by message. Therefore the minimal

Line 66:

Line 78:

the used sections.

the used sections.

+

<pre>

|----- time ------| |type| |code| |-value-|

|----- time ------| |type| |code| |-value-|

8c66 4819 721c 0006 0002 0002 03a8 0000

8c66 4819 721c 0006 0002 0002 03a8 0000

Line 73:

Line 86:

8c66 4819 9a50 0006 0002 0002 0396 0000

8c66 4819 9a50 0006 0002 0002 0396 0000

8c66 4819 9a57 0006 0000 0000 0000 0000

8c66 4819 9a57 0006 0000 0000 0000 0000

+

</pre>

+

Using recent kernels (>=2.6.29), you might see this instead, because accelerometers are now reporting

+

absolute values instead of relative ones (more on that later) :

+

+

<pre>

+

|----- time ------| |type| |code| |-value-|

+

8163 49da 6d62 000d 0000 0000 0000 0000

+

8163 49da 91d8 000d 0003 0000 0048 0000

+

8163 49da 9231 000d 0003 0001 0012 0000

+

8163 49da 9251 000d 0003 0002 03ba 0000

+

8163 49da 9270 000d 0000 0000 0000 0000

+

8163 49da b6cf 000d 0003 0000 0036 0000

+

</pre>

−

I think the time structure does not need further explaination. A lot more

+

I think the time structure does not need further explanation. A lot more

interesting is the type, code and value part of every message. Lets take a

interesting is the type, code and value part of every message. Lets take a

closer look at these parts.

closer look at these parts.

Line 95:

Line 121:

The definition should not be taken too seriously in this context, because the

The definition should not be taken too seriously in this context, because the

data values provided by the accelerometer always represent the absolute

data values provided by the accelerometer always represent the absolute

−

acceleration measured at the given time.

+

acceleration measured at the given time. So, in newer kernels, the proper

+

''absolute movement'' notification is used instead :

+

+

;0x03:

+

:This event is called ''EV_ABS'' and signals ''absolute movement''. It replaces the ''EV_REL'' notifications in newer kernels, but

+

the information (the event codes) is the same.

Line 104:

Line 135:

−

====Syncronization event codes====

+

====Synchronization event codes====

−

The syncronization event may use quite a lot of codes, as ''linux/input.h''

+

The synchronization event may use quite a lot of codes, as ''linux/input.h''

shows. However the only used one seems to be the 0x00 code.

shows. However the only used one seems to be the 0x00 code.

;0x00:

;0x00:

−

:This code is refered to as SYN_REPORT. It means that the last dataset was completely transmitted. Therefore the before transmitted set of data values can be considered complete. This means if this message is recieved you may process the given data further.

+

:This code is referred to as SYN_REPORT. It means that the last dataset was completely transmitted. Therefore the before transmitted set of data values can be considered complete. This means if this message is received you may process the given data further.

−

====Relative movement event codes====

+

===Relative movement event codes===

The amount of possible codes for this event type is quite big. The only used

The amount of possible codes for this event type is quite big. The only used

Line 129:

Line 160:

The X, Y and Z axis are to be understand as defined in the chapter about [[#Axis_orientation|Axis orientation]].

The X, Y and Z axis are to be understand as defined in the chapter about [[#Axis_orientation|Axis orientation]].

Although the information is the same as before, the absolute interface differs from the relative one by the fact that

+

only changed values are reported. So if the phone was completely idle, experiencing exactly the same acceleration from

+

one sample to the next, you will read only synchronization events (this will not happen due to noise, though).

+

+

So when you miss the value for an axis between two synchronization events, just assume the previous value still holds.

==Test application==

==Test application==

A simple test application written in ruby:

A simple test application written in ruby:

+

<pre>

#!/usr/bin/env ruby

#!/usr/bin/env ruby

x = 0

x = 0

Line 163:

Line 209:

z = 0

z = 0

File.open("/dev/input/event3") do |f|

File.open("/dev/input/event3") do |f|

−

while true

+

while true

−

event = f.read(16).unpack("llSSl")

+

event = f.read(16).unpack("llSSl")

−

time = event[0] + event[1] / 1000000.0

+

time = event[0] + event[1] / 1000000.0

−

type = event[2]

+

type = event[2]

−

code = event[3]

+

code = event[3]

−

value = event[4]

+

value = event[4]

−

if type == 2

+

if type == 2 || type == 3

−

x = value if code == 0

+

x = value if code == 0

−

y = value if code == 1

+

y = value if code == 1

−

z = value if code == 2

+

z = value if code == 2

−

end

+

end

−

if type == 0 && code == 0

+

if type == 0 && code == 0

−

sum = Math::sqrt(x*x + y*y + z*z).to_i

+

sum = Math::sqrt(x*x + y*y + z*z).to_i

−

printf("%5d %5d %5d %5d\n", x, y, z, sum)

+

printf("%5d %5d %5d %5d\n", x, y, z, sum)

−

end

+

end

−

end

+

end

end

end

+

+

+

</pre>

Equivalent version in python:

Equivalent version in python:

Line 198:

Line 247:

event = in_file.read(16)

event = in_file.read(16)

while event:

while event:

−

(time1,time2, type, code, value) = \

+

(time1,time2, type, code, value) = \

−

struct.unpack(fmt,event)

+

struct.unpack(fmt,event)

−

time = time1 + time2 / 1000000.0

+

time = time2 / 1000.0

−

if type == 2:

+

if type == 2 or type == 3:

−

if code == 0:

+

if code == 0:

−

x = value

+

x = value

−

if code == 1:

+

if code == 1:

−

y = value

+

y = value

−

if code == 2:

+

if code == 2:

−

z = value

+

z = value

−

if type == 0 and code == 0:

+

if type == 0 and code == 0:

−

sum = int(sqrt(x*x + y*y + z*z))

+

sum = int(sqrt(x*x + y*y + z*z))

−

print x, y, z, sum

+

print time, x, y, z, sum

−

event = in_file.read(16)

+

event = in_file.read(16)

in_file.close()

in_file.close()

+

+

</pre>

+

+

Equivalent in Perl

+

+

<pre>

+

#!/usr/bin/perl

+

+

$x = 0;

+

$y = 0;

+

$z = 0;

+

+

$secondsensorfile = "/dev/input/event3";

+

#open file in binary mode

+

open FILE, $secondsensorfile;

+

binmode FILE;

+

+

while (read FILE, $buf,16) {

+

($t1,$t2,$type,$code,$value)=unpack "iissi",$buf;

+

if ($type==2) {

+

if ($code==0) {$x=$value};

+

if ($code==1) {$y=$value};

+

if ($code==2) {$z=$value};

+

}

+

if (($type==0) && (code==0)) {

+

printf "%f %05d %05d %05d\n",$t1+$t2/1000000,$x,$y,$z;

+

}

+

}

+

+

</pre>

+

+

A PHP absolute version

+

+

<pre>

+

<?php

+

+

$x = 0;

+

$y = 0;

+

$z = 0;

+

+

$secondsensorfile = "/dev/input/event3";

+

#open file in binary mode

+

$dfeed2 = fopen($secondsensorfile, "rb");

+

+

while (!feof($dfeed2)) {

+

$contents = fread($dfeed2, 16);

+

$arr = unpack("s4time/S1data/S1code/l1value",$contents);

+

if ($arr[data]=3){

+

if ($arr[code]==0) $x=$arr[value];

+

if ($arr[code]==1) $y=$arr[value];

+

if ($arr[code]==2) $z=$arr[value];

+

}

+

print "x= $x, y= $y, z=$z \n";

+

}

+

fclose($handle);

+

?>

+

+

</pre>

+

+

Also, for the new absolute interface (Perl) :

+

+

<pre>

+

#!/usr/bin/perl

+

+

$x = 0;

+

$y = 0;

+

$z = 0;

+

+

$secondsensorfile = "/dev/input/event3";

+

#open file in binary mode

+

open FILE, $secondsensorfile;

+

binmode FILE;

+

+

while (read FILE, $buf,16) {

+

($t1,$t2,$type,$code,$value)=unpack "iissi",$buf;

+

if ($type==3) {

+

if ($code==0) {$x=$value};

+

if ($code==1) {$y=$value};

+

if ($code==2) {$z=$value};

+

}

+

if (($type==0) && ($code==0)) {

+

printf "%f %05d %05d %05d\n",$t1+$t2/1000000,$x,$y,$z;

+

}

+

}

+

</pre>

+

+

Read the data in C (FIXME: We're not printing times here).

+

+

<pre>

+

#include <stdio.h>

+

#include <time.h>

+

#include <sys/times.h>

+

#include <sys/stat.h>

+

#include <fcntl.h>

+

#include <unistd.h>

+

#include <stdint.h>

+

#include <assert.h>

+

+

struct input_event {

+

struct timeval time;

+

uint16_t type;

+

uint16_t code;

+

int32_t value;

+

};

+

+

int read_all(int fd, char *buf, int count)

+

{

+

int n_read = 0;

+

while (n_read != count) {

+

int result = read(fd, buf + n_read, count - n_read);

+

if (result < 0)

+

return result;

+

else if (result == 0)

+

return n_read;

+

n_read += result;

+

}

+

return n_read;

+

}

+

+

int main(int argc, char *argv[])

+

{

+

int fd;

+

struct input_event ev;

+

+

assert(16 == sizeof(struct input_event));

+

+

if (argc != 2) {

+

fprintf(stderr, "missing /dev/input/XXX\n");

+

return 1;

+

}

+

+

+

+

while (1) {

+

if ((fd = open(argv[1], O_RDONLY)) == -1) {

+

perror("open");

+

return 1;

+

}

+

int ret = read_all(fd, (char *) &ev, sizeof(struct input_event));

+

if (ret != sizeof(struct input_event)) {

+

fprintf(stderr, "ret == %d\n", ret);

+

perror("read");

+

return 1;

+

}

+

close(fd);

+

printf("type:%u code:%u value:%d\n", ev.type, ev.code, ev.value);

+

usleep(500000); /* 2 reads per second, you might want to read more */

+

}

+

+

return 0;

+

}

+

</pre>

+

+

And in Guile Scheme. (This one is a complete autorotation program.)

+

+

<pre>

+

;; Open device file for the second accelerometer (whose axes are

+

;; aligned with the screen). Must make Guile's internal buffer small

+

;; (16), otherwise it reads too far ahead and this program gets mostly

+

;; out of date data.

+

(define e (open-file "/dev/input/event3"

+

"r0"))

+

(setvbuf e _IOFBF 16)

+

+

;; Uniform vector for reading data from device file. Must match

+

;; sizeof(struct input_event), otherwise the read can get an 'invalid

+

;; argument' error.

+

(define v (make-u16vector 8))

+

+

(define (current-acceleration)

+

(let loop ((x #f) (y #f) (z #f))

+

(uniform-vector-read! v e)

+

(let ((code (u16vector-ref v 4))

+

(axis (u16vector-ref v 5))

+

(meas (+ (u16vector-ref v 6)

+

(* 65536 (u16vector-ref v 7)))))

+

(if (>= meas (expt 2 31))

+

(set! meas (- meas (expt 2 32))))

+

(case code

+

((2 3)

+

(case axis

+

((0) (loop meas y z))

+

((1) (loop x meas z))

+

((2) (loop x y meas))

+

(else (loop x y z))))

+

(else

+

(if (and x y z)

+

(list x y z)

+

(loop x y z)))))))

+

+

(define (orientation x y z)

+

(cond ((and (< y -200)

+

(> (- y) x y))

+

0)

+

((and (> x 200)

+

(< (- x) y x))

+

1)

+

((and (> y 200)

+

(< (- y) x y))

+

2)

+

((and (< x -200)

+

(> (- x) y x))

+

3)

+

(else #f)))

+

+

(define (main)

+

(let loop ((ori #f))

+

(let ((new-ori

+

(apply orientation

+

(current-acceleration))))

+

(and ori

+

new-ori

+

(not (= ori new-ori))

+

(system* "xrandr"

+

"-o"

+

(number->string new-ori)))

+

(sleep 1)

+

(loop (or new-ori ori)))))

+

+

(main)

</pre>

</pre>

== The /sys interface ==

== The /sys interface ==

−

To get the sampling rate of the first accelerometer ''spi0.0'' :

+

To get the sampling rate of the first accelerometer ''lis302dl.1'' :

−

# cat /sys/bus/spi/drivers/lis302dl/spi0.0/sample_rate

+

# cat /sys/devices/platform/lis302dl.1/sample_rate

100

100

To set the sampling rate :

To set the sampling rate :

−

# echo 400 > /sys/bus/spi/drivers/lis302dl/spi0.0/sample_rate

+

# echo 400 > /sys/devices/platform/lis302dl.1/sample_rate

−

# cat /sys/bus/spi/drivers/lis302dl/spi0.0/sample_rate

+

# cat /sys/devices/platform/lis302dl.1/sample_rate

400

400

−

The other one is ''spi0.1''. See [[GTA02 sysfs#Accelerometers]] for the other options.

+

+

OM2008.12 has too big threshold for some games and applications (value=18).

+

You can use sysfs to change it. Use command like this one:

+

+

# echo 10 > /sys/devices/platform/lis302dl.1/threshold

+

+

+

+

The other one is ''lis302dl.2''. See [[GTA02 sysfs#Accelerometers]] for the other options.

+

+

+

'''NOTE''' : --[[User:Garthps|Garthps]] 15:38, 14 December 2010 (UTC) : the interface has changed in newer kernels. Now it is in :

+

+

/sys/devices/platform/spi_s3c24xx_gpio.0/spi3.0/

+

+

and

+

+

/sys/devices/platform/spi_s3c24xx_gpio.0/spi3.1/

+

+

'''NOTE2''' : --[[User:Boudewijn|Boudewijn]] 10:42, 11 August 2011 (UTC): It might have changed once more; I could not find it using om-gta02 2.6.37.6 #1 Fri Jun 10 18:09:48 CEST 2011 armv4tl, but found it in:

+

/sys/class/i2c-adapter/i2c-0/0-0073/lis302dl.1/sample_rate

+

(following the path suggested by [[GTA02_sysfs#new:_.2Fsys.2Fclass.2Fi2c-adapter.2Fi2c-0.2F0-0073.2Fspi_s3c24xx_gpio.0.2Fspi3..7B0.7C1.7D.2F|sysfs for 2.6.28]])

* [http://www.st.com/stonline/products/literature/ds/12726/lis302dl.htm] The interesting hardware spec of the accellerometer built into the gta02, where you can learn how this device is actually functioning on the low-level side.

To understand the values provided by the accelerometers it is crucial to
understand how the sensors are oriented. In the following I will refer to the
sensors as first and second sensor. The way to access the data sources of
these two will be described later on in this document.

To explain the axis orientation I have created some images for easier
understanding. Note that X and Y are in the same plane as the screen, while the Z arrow is pointing "into the screen", i.e. behind the Neo. The device for the first sensor is /dev/input/event2.

In contrast to the first sensor the second one is turned 45 degrees around the
Z axis. See the attached image to get a clue about its orientation. Note that X and Y are in the same plane as the screen, while the Z arrow is pointing "into the screen", i.e. behind the Neo. The device for the second sensor is /dev/input/event3.

These device nodes can be opened by the default filesystems calls for reading
files. The data can be read from the stream as soon as the sensor measures it. Note that since the data is exported as normal files you can easily e.g. use netcat to stream the accelerometer data over bluetooth to your laptop to a control a game running on your laptop.

To be able to use the measured data it is important to understand the format
in which the data is provided.
The structure of the given data is based on the kernel input event message
system which exports the following data structure:

The types categorize the incoming messages. All possible types can be found in
the kernel sources or include files in INCLUDEDIR/linux/input.h
During the tests with the accelerometers I observed the fact that only two
different message types are used.

0x00

According to linux/input.h this event is called EV_SYN. It signals the wish to syncronize. Normally this event is used in combination with code 0x00 to mark the send data complete and therefore applyable.

0x02

This event is called EV_REL and signals relative movement. It is used to transmit the acceleration the sensors encounter.

The definition should not be taken too seriously in this context, because the
data values provided by the accelerometer always represent the absolute
acceleration measured at the given time. So, in newer kernels, the proper
absolute movement notification is used instead :

0x03

This event is called EV_ABS and signals absolute movement. It replaces the EV_REL notifications in newer kernels, but

The synchronization event may use quite a lot of codes, as linux/input.h
shows. However the only used one seems to be the 0x00 code.

0x00

This code is referred to as SYN_REPORT. It means that the last dataset was completely transmitted. Therefore the before transmitted set of data values can be considered complete. This means if this message is received you may process the given data further.

Although the information is the same as before, the absolute interface differs from the relative one by the fact that
only changed values are reported. So if the phone was completely idle, experiencing exactly the same acceleration from
one sample to the next, you will read only synchronization events (this will not happen due to noise, though).

So when you miss the value for an axis between two synchronization events, just assume the previous value still holds.

Views

Personal tools

Scope

This document describes a way to access the data provided by the
accelerometers. Furthermore the format of the acquired datastream is explained
in detail. This document does not cover the physical basics of acceleration or
the mathematical details on this subject.

Axis orientation

To understand the values provided by the accelerometers it is crucial to
understand how the sensors are oriented. In the following I will refer to the
sensors as first and second sensor. The way to access the data sources of
these two will be described later on in this document.

The Z axis is pointing from the display downwards to to the back of the
openmoko. This applies to both of the sensors.

Orientation of the first sensor

To explain the axis orientation I have created some images for easier
understanding.

Orientation of the second sensor

In contrast to the first sensor the second one is turned 45 degrees around the
Z axis. See the attached image to get a clue about its orientation.

Data acquisition

The information from both of the accelerometers is exported through two
different input event based file mappings. These device nodes can be found at
/dev/input/event2 and /dev/input/event3.

The sensor I am refering to as the first one is event2. Therefore the second
one is accessible through the event3 device.

These device nodes can be opened by the default filesystems calls for reading
files. The data can be read from the stream as soon as the sensor measures it.

Data structure

To be able to use the measured data it is important to understand the format
in which the data is provided.
The structure of the given data is based on the kernel input event message
system which exports the following data structure:

I think the time structure does not need further explaination. A lot more
interesting is the type, code and value part of every message. Lets take a
closer look at these parts.

Event types

The types categorize the incoming messages. All possible types can be found in
the kernel sources or include files in INCLUDEDIR/linux/input.h
During the tests with the accelerometers I observed the fact that only two
different message types are used.

0x00

According to linux/input.h this event is called EV_SYN. It signals the wish to syncronize. Normally this event is used in combination with code 0x00 to mark the send data complete and therefore applyable.

0x02

This event is called EV_REL and signals relative movement. It is used to transmit the acceleration the sensors encounter.

The definition should not be taken too seriously in this context, because the
data values provided by the accelerometer always represent the absolute
acceleration measured at the given time.

Event codes

Both types of event may supply different codes. These codes can be understood
as some kind of further specification about the specified data values

Syncronization event codes

The syncronization event may use quite a lot of codes, as linux/input.h
shows. However the only used one seems to be the 0x00 code.

0x00

This code is refered to as SYN_REPORT. It means that the last dataset was completely transmitted. Therefore the before transmitted set of data values can be considered complete. This means if this message is recieved you may process the given data further.

Relative movement event codes

The amount of possible codes for this event type is quite big. The only used
ones are the following ones.

0x00

REL_X - Acceleration in x direction

0x01

REL_Y - Acceleration in y direction

0x02

REL_Z - Acceleration in z direction

The X, Y and Z axis are to be understand as defined in the chapter about Axis orientation.

A typical message block

A typical message block consists of 3 messages containing the acceleration
data for every of the three axis followed by a syncronization message to
signal the end of the block.

The following example is such a message block with detailed explanation of its
different messages and data sections.